In continuous casting processes of the molten metal, in order to prevent heat dissipation and oxidation of molten metal, it is necessary to cover the molten metal with the right amount of slag powder which forms a flux slag layer. However, the level of the liquid metal is also a critical factor in many processes. For example, the liquid steel level in a tundish plays a role in determining the cast-on time, preventing the flux slag from getting into billet/slab and improving the melt-to-shop ratio. It is hard to measure exactly the liquid steel level, as a slag layer cover the surface of liquid steel and the slag layer thickness is uncertain.
As a conventional method of measuring the liquid metal level such as liquid steel level, a publicly known arrangement is the weighing method in which the calculation and determination of the liquid steel level is based on weighing out the total weight of the tundish and the molten steel (including slag layer) and some known parameters such as the weight and cubage of the tundish, and liquid steel density, etc. In such method, however, it was unavoidable that the calculated liquid level is often inaccurate since the thickness of the slag layer has also been included in the measurement value of the liquid steel level due to uncertain slag weight. Further measuring deviation of the liquid steel level is also inevitable since the tundish lining can be eroded inch by inch by molten steel resulting in an irregular change of the capacity of the tundish. Namely, it is almost impossible to determine the accurate correspondence relation between the change of the total weight of the tundish and the liquid steel level, so the liquid level of molten metal can hardly be calculated accurately
A method of measuring the liquid level of molten steel in the ladles has been published, as disclosed in Ger Pat. No. DE2945251-A1 “Measuring level of liquid steel in ladles etc.—using laser beam directed at surface at angle to axis of imaging system”. In this patent, a laser beam is incident upon the surface of slag layer floating on the molten steel so as to form a laser bright spot therein. The position height of the laser bright spot is detected by the imaging system as a measurement of liquid level of molten steel, and the displacement of the laser spot in vertical direction indicates a variation of the level of the liquid steel. In the method, however, there still remains the problem that the measurement value of molten steel level is inaccurate since an uncertain thickness of the slag layer has likewise been included in the measurement value. Further, an error is produced in a measurement value due to an uneven surface of slag layer.
The aforementioned prior art cannot determine the thickness values of slag layer floating on molten metal, so that the liquid level of molten metal cannot be measured accurately and stably. Hence, there is a strong need for a method and an apparatus for accurately measuring the thickness of slag layer and then measuring the liquid level of molten metal such as molten steel.